Feeding by the mixotrophic thecate dinoflagellate Fragilidium cf. mexicanum on red-tide and toxic dinoflagellates

We investigated prey species, prey selectivity, growth rates, grazing rates , and the effect of light and nutrient environment on feeding for the theca te mixotrophic dinoflagellate Fragilidium cf. mexicanum. Among the red-tide and toxic dinoflagellate prey offered, Lingulodinium polyedrum, Gymnodiniu m sanguineum, Prorocentrum micans, P. minimum, and Scrippsiella trochoidea were ingested by F. cf. mexicanum, but Amphidinium carterae and Cochlodiniu m polykrikoides were not. The feeding frequency (FF), based on the percent of F. cf. mexicanum containing one or more target prey cells, was significa ntly affected by prey species. The maximum FFs of F. cf, mexicanum on L. po lyedrum and S. trochoidea after 72 h incubation in a unialgal diet (50 and 89%, respectively) were much higher than those for P. micans, P. minimum, a nd G. sanguineum (6, 10, and 2%, respectively). FFs on L. polyedrum and S. trochoidea were significantly affected by prey concentration, but those on P. micans and G, sanguineum were not. F. cf. mexicanum strongly selected L. polyedrum over S. trochoidea in prey mixtures. With increasing mean prey c oncentration, growth and ingestion rates of F. cf. mexicanum feeding on L. polyedrum increased, with saturation at a mean prey concentration of approx imately 500 cells ml(-1). The maximum specific growth rate (mixotrophic gro wth) of F. cf. mexicanum on L. polyedrum was 0.36 d(-1), under a 12 h light :12 h dark cycle of 20 mu E m(-2) s(-1), while its growth rate (phototrophi c growth) under the same light conditions without added prey was -0.05 d(-1 ). The maximum ingestion rate of F. cf. mexicanum on L. polyedrum, 3.9 prey eaten predator(-1) d(-1), was comparable to those of the co-occurring hete rotrophic dinoflagellates Protoperidinium cf. divergens and P. crassipes fo r the same prey. However, maximum clearance rate of F. cf. mexicanum, 6 mu l predator h(-1), was much higher than those of P. cf. divergens and P. cra ssipes. The ingestion rate of F. cf. mexican um on L. polyedrum was not sig nificantly affected by light intensity or nutrient concentration when prey was plentiful.